This invention relates to the cleaning of contaminated surfaces, and is particularly concerned with the removal of polymeric surface coatings which have been altered or changed by ultraviolet light exposure, without removing the basic coating. The invention also is directed to removal of contaminants from metal surfaces.
In the aircraft industry, component or detail parts coated with an organic polymeric coating, such as polyurethane, e.g. integral fuel tank coatings of this type, are subjected to ultraviolet light rays from windows, artificial lighting or temporary exterior exposure for extended periods prior to assembly, resulting in a change in the composition of the surface of the coating. During assembly, scratches, gouges and chipping occur, frequently causing damage down to the base metal surface. These surfaces are solvent cleaned with conventional solvents such as 1,1,1-trichlorethane or methyl ethyl ketone, prior to being touched up with fresh coating. However, during service, gross adhesion failures of the touchup coating have occurred in many aircraft. The resultant effect is reduced corrosion resistance of the damaged area and possible contamination of the fuel filter by the loose coating.
U.S. Pat. No. 3,796,602 discloses stripping or removal of polymer resist masks by treatment at elevated temperatures employing a blend of glycol ether, surfactant and an alkaline agent.
U.S. Pat. No. 4,078,102 discloses stripping of organic polymer resist layers by treating with a mixture of an aldehyde or a ketone, an alcohol and an alkali metal hydroxide or carbonate.
U.S. Pat. No. 2,294,479 discloses obtaining a high gloss on coatings by exposing the coatings with a vapor bath from a heated solvent mixture of ketones, alcohol and acetates.
U.S. Pat. Nos. 2,228,108 and 2,262,488 disclose a solvent blend for duplicating ink comprising an alcohol and a glycol ether.
However, applicants have found that commercially available solvents and solvents of the types disclosed in the above patents are ineffective for removal of ultraviolet changed polymeric surface coatings, particularly the polyurethane and epoxy type polymer coatings, employed on aircraft component parts, such as integral fuel tank coatings, and which have been oxidized by ultraviolet light, and for proper adhesion of an overcoating to the treated surface.
An object of the present invention accordingly, is to provide procedure and solvent compositions which are effective for removing polymeric surface coatings which have been altered by previous exposure to ultraviolet light rays.
Another object is to provide a process and solvent compositions for removal of ultraviolet light changed polymeric surface coatings without degrading or removing the base coating.
Still another object is the provision of procedure and solvent compositions for removing ultraviolet light changed polymer coatings, particularly polyurethane and epoxy coatings, while enhancing the adhesion of a touch-up coating or overcoating subsequently applied to the base coating.
Other objects and advantages of the invention will appear hereinafter.